This invention relates to a method and apparatus for making an electric transformer core that comprises thin superposed strips of amorphous metal arranged in groups and surrounding the window of the core. The invention relates more particularly to a method and apparatus for making a core of this type that is characterized by lap joints between the opposite ends of each of these groups.
A widely-used type of lap joint construction that has good magnetic properties is one in which the lap joints are angularly offset, or staggered, repeating in a stairstep fashion as one proceeds from the window to the outer periphery of the core. This type of construction is referred to herein as a step-lap, or distributed-lap, joint construction. Example of this117 type construction are illustrated in our U.S. Pat. No. 4,734,975 and in U.S. Pat. No. 4,741,093--Lee and Ballard, both of which are incorporated by reference in the present application. A disadvantage of this type of joint construction is that its use produces an extra build-up in the cross-sectional area of the core in the joint region, and this build-up typically appears as a "bump" projecting radially outwardly on the outer surface of the core. This bump tends to produce significant problems in the manufacture of the core, as will soon be described. The bump can be eliminated if the core employs so-called "short sheets", utilizing a short sheet each time the step pattern of the lap joints is repeated. Each of these short sheets is a partial-length lamination having one of its ends butted with the overlapping end of the last lamination of one step-lap joint pattern and the other of its ends butted with the underlapping end of the first lamination of the next step-lap pattern. The presence of these short sheets builds up the cross-section of the rest of the core to equal the cross-section of the joint region, thus eliminating the above-described "bump". But for reasons well known in the art, as explained, for example, in the foresaid U.S. Pat. No. 4,741,096--Lee and Ballard, by the presence of short sheets results in localized regions of high flux density which can produce undesirable saturation effects. We, therefore, avoid the "short-sheet" approach in constructing our core and utilize a different approach for eliminating, or at least significantly reducing the size of, the above-described outwardly projecting bump during the portion of the core-making process when such bump can cause significant manufacturing problems.
Some of the problems associated with the above-described outwardly-projecting bump are as follows. If the core is to be assembled from superposed thin strips of amorphous metal, the presence of the bump makes it very difficult to effectively guide and locate the edges of the strips during a conventional core assembly operation, e.g., one in which the amorphous strips are wrapped about a rotating arbor with assistance from a moving belt partially surrounding the arbor. Another problem resulting from the presence of the bump in such a core assembly operation is that the increasingly eccentric mass of the core form as it is built-up around the arbor limits the speed at which the arbor can be rotated, thereby limiting the speed of the assembly operation. Still another problem is the tendency for laminations to change angular position as the arbor rotates. In this latter respect, it is difficult to keep the inside arbor and the outside wrapping belt moving at the same angular speed, especially as the belt contacts the bump.
Another problem that is encountered when one attempts to construct a core of amorphous metal strips encircling the core window is that because the amorphous metal strips are very thin (typically only about 1 mil in thickness, which is only about 1/10 to 1/20 the thickness of conventional silicon steel strips typically used), a very large number of strips must be wrapped or otherwise assembled about the core window in order to achieve the desired build of the core. Individually wrapping this large number of strips about the core window would be an excessively time-consuming and expensive process. To avoid the need for individually wrapping this large number of strips, it has been proposed, for cores with lap joints, that the strips be simultaneously wrapped about the core window in groups individually made up of the number of strips suitable for one lap joint, e.g., 10 to 20 strips. It would be desirable if the strips could be simultaneously wrapped in much larger numbers, thus forming a plurality of lap joints, and in the case of the step-lap joint core, a plurality of lap joints offset by precise predetermined amounts. Using conventional methods of core assembly, it is difficult to simultaneously wrap, or otherwise assemble, this many amorphous strips with their ends precisely located to provide the desired precisely located step-lap joints.
One way of ameliorating some of the above-described problems of precisely locating the strips is to wet the strips prior to core assembly with a suitable liquid. The liqid tends to hold adjacent strips together through surface tension during assembly, blocking undesired displacement of the strips. Unfortunately, the use of such liquids may involve environmental problems, or could cause rusting of the amorphous metal, particularly if the liquid is not fully evaporable during the core-making process. It is therefore desirable to eliminate the need for such liquids during the core assembly process.